A thin film corresponds to a layer of material deposited on a solid support or substrate, wherein the layer ranges in thickness from fractions of a nanometer (monolayer) to several micrometers. Thin films are employed, for example, in electronics (e.g., insulators, semiconductors, or conductors for integrated circuits), optical coatings (e.g., reflective, anti-reflective coatings, or self-cleaning glass) and packaging (e.g., aluminum-coated PET film).
Thin film deposition may be accomplished using a variety of gas phase chemical and/or physical vapor deposition techniques. Many of these deposition techniques are able to control layer thickness within a few tens of nanometers. Thin film deposition is also achieved by liquid phase and electrochemical techniques where the thickness of the final film is not well controlled. Examples include copper deposition by electroplating and sol gel deposition.
Gas phase deposition techniques fall into two broad categories, depending on whether the process is primarily chemical or physical. In a chemical deposition process, a precursor undergoes a chemical change at a solid surface, leaving a solid layer on the surface. In a chemical vapor deposition (CVD) process, a gas-phase precursor, often a halide or hydride of the element to be deposited, reacts with a substrate on the surface, leading to formation of the thin film on the surface. Atomic layer deposition (ALD) is a thin film growth technique based on sequential, self-limiting surface reactions (George, 2010, Chem Rev 110:111-131). ALD can deposit extremely conformal thin films with atomic layer control. ALD has developed rapidly over the last 10-15 years to meet many industrial needs such as the miniaturization of semiconductor devices. ALD can deposit a wide range of materials from metal oxides to metals (Miikkulainen, et al., 2013, J Appl Phys 113). ALD is typically accomplished using thermal chemistry. However, sometimes plasma ALD is employed to enhance the surface reactions.
In contrast, atomic layer etching (ALE) is a thin film removal technique based on sequential, self-limiting surface reactions (Agarwal & Kushner, 2009, J. Vacuum Sci & Tech A 27:37-50; Athavale & Economou, 1995, J Vacuum Sci & Tech A—Vacuum Surfaces and Films 13:966-971; Athavale & Economou, 1996, J Vacuum Sci Tech B 14:3702-3705). ALE can be viewed as the reverse of ALD. ALE should be able to remove thin films with atomic layer control. Compared with the large number of ALD processes, ALE processes have not been defined for as many materials. In addition, no thermal chemical processes have been demonstrated for ALE. ALE processes that have been reported have used excitation such as ion-enhanced or energetic noble gas atom-enhanced surface reactions. Most of the documented ALE processes have adsorbed a halogen on the surface of the material. Ion or noble gas atom bombardment is then used to desorb halogen compounds that etch the material.
Developing thermal self-limiting ALE reactions that are the reverse of ALD reactions is difficult. ALD reactions are typically exothermic reactions that are favorable thermochemical reactions. These thermal reactions are spontaneous with negative AG values (G is the Gibbs free energy). Performing ALD reactions in reverse should not be possible because of these thermodynamic considerations. The challenge for thermal ALE reactions is to find alternative, self-limiting, reactions with different reactants that are exothermic and display negative AG values to ensure a spontaneous reaction.
There is a need in the art for novel methods of performing atomic layer etching (ALE) on a surface. Such methods should be self-limiting and allow for atomic level precision. The present invention meets this need.